Various types of multitubular cartridges having high surface area and low hold up volume, such as hollow fibers, will be coated with L-phenylalinine degrading enzymes. These will be tested in vitro and in vivo for kinetic behavior, stability and blood compatibility in experimental animals, including monkeys, with and without phenylalanine overload. The aim of the study is to assess the clinical potential of such devices for in vivo phenylalanine degradation and establish optimum design and operating conditions for short and long term application with extracorporeal shunts. If successful, such devices may be useful adjuncts to the treatment of patients with phenylketonuria, especially when diet is discontinued and temporary overload of phenylalanine may occur. These studies may serve as models for developing treatments for other metabolic disorders with novel immobilized enzyme systems.